Cataracts in horses are cloudy areas within the lens of the eye, which is responsible for directing light to the retina. [1]

This lens must be clear to allow light to pass through and enable visual perception. Depending on the severity of cloudiness, cataracts can cause partial or full blindness in the horse’s affected eye.

Most cases of equine cataracts result from equine recurrent uveitis (ERU), a disorder where the immune system attacks structures within the eye. Other causes of cataracts in horses include traumatic injuries or abnormal lens development during gestation.

The only treatment available for cataracts is surgical removal of the lens. Surgery is typically reserved for horses with severe cataracts resulting in complete blindness. Removal of the cataract can restore some vision and allow horses to return to their typical lifestyle.

Causes of Cataracts in Horses

Cataracts are a serious eye condition characterized by cloudiness or opacity in the lens of the eye, which can impact a horse’s vision.

The lens is normally transparent, allowing light to focus properly on the retina for vision. When cataracts form, this transparency is compromised, resulting in blurred or distorted vision.

Cataracts can either be congenital (present at birth) or acquired (develop during the horse’s lifetime). [1]

Congenital Cataracts

Congenital cataracts are the most common eye abnormality present in foals at birth. [1] Around 0.5% of foals have eye abnormalities at birth, with cataracts making up around 35% of all such abnormalities. [1][2]

In some cases, the lenses in affected foals appear normal at birth, but quickly develop cataracts during the first few months of life. [1] In these foals, it is likely the abnormality is congenital but the defect may not have been large enough for identification at birth. [1]

Genetic Inheritance

Certain breeds have a predisposition to congenital cataracts, including: [1][3][4]

Congenital cataracts in these breeds are inherited, meaning that a genetic mutation in the parents causes the condition. [3]

Spontaneous Congenital Cataracts

In other breeds, congenital cataracts can occur spontaneously due to poor formation of the lens during development of the foal. [3] Possible causes of spontaneous congenital cataracts include: [1]

  • Traumatic injuries
  • Poor nutrition
  • In utero infections
  • Exposure to toxins
  • Exposure to radiation

In most cases, the underlying cause of spontaneous congenital cataracts is unknown. [1]

Acquired Cataracts

Most cataracts in horses develop in association with equine recurrent uveitis (ERU), an inflammatory disease within the eye. [1] Around 8% of all horses have equine recurrent uveitis, making ERU the most common cause of cataracts in horses. [3]

In horses with ERU, the immune system attacks structures of the eye, resulting in damage that can affect lens function. [1] A unique feature of the disease is recurrence, where horses repeatedly develop symptoms of uveitis such as squinting, tearing, and swelling of the eye. [5]

Common risk factors for developing ERU include: [5]

Cataracts can also occur due to aging, especially in horses over 20 years old. [1] Senile cataracts develop over time due to deterioration of the lens fibres. [1]

Traumatic injuries to the eye can also cause cataracts if the lens is damaged. [1]

Equine Eye Anatomy

Given the high rates of occurrence of eye injury and disease in horses, it can be helpful for horse owners to familiarize themselves with eye anatomy.

horse-eye-anatomy | Mad Barn USAIllustration:

Symptoms

Most horses show no symptoms of cataracts unless examined by a veterinarian. [6] Therefore, cataracts are most commonly diagnosed during routine checkups, newborn foal examinations, or prepurchase exams. [6]

When symptoms are present, the most common signs of cataracts are blindness and cloudiness of the eye. [1] Horses that show these symptoms typically have very severe cataracts that affect most of the lens. [6]

Other symptoms that can indicate deteriorating vision in horses include: [7]

  • Headshaking
  • Spooking or shying
  • Behavioural changes, particularly in herd situations
  • Hesitancy in unfamiliar situations
  • Bumping into objects or obstacles
  • Difficulty with transitioning from light to dark areas or vice versa
  • Decreased vision in dim lighting conditions
Obvious signs of vision loss in horses require emergency veterinary attention. Urgent medical intervention provides the best chance of preserving vision.

Diagnosis

Veterinarians diagnose cataracts using a specialized ophthalmic examination. [6] Medications that dilate the horse’s pupils may be necessary to thoroughly evaluate the lens. [6]

During examination of the cataract, the veterinarian notes several features that may impact prognosis. [6] These features include: [6]

  • Location within the lens
  • Size and density of the cataract
  • Presence of other ocular abnormalities
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Treatment

Currently, the only available treatment for cataracts is surgical removal of the lens. [1] Referral to a specialist may be required.

Most veterinary ophthalmologists recommend surgical intervention in foals with cataracts, as restoring vision early on allows the foal’s visual centers to compensate for their absent lens. [8] Foals also have a lower risk of complications compared to adult horses due to their smaller eyes and rapid healing times. [8]

In adult horses, surgical removal of cataracts is usually only recommended in severe cases where there is visual impairment impacting quality of life or ability to work. [1] There are several reasons adult horses are poor candidates for cataract surgery, including: [1][8]

  • Large eye size which most surgical equipment is not designed for
  • Association between cataracts and uveitis, which can prevent healing
  • Requirement for extensive topical eye medication use after surgery, which many horses do not tolerate
  • Risk of injury to riders or handlers after surgery due to reduced vision
  • Increased risk of general anesthesia in adult horses

Horses with smaller cataracts may benefit from medications to dilate their pupils, providing improved vision around the cataract. [1] These medications may allow horses to avoid surgical treatment entirely if the cataract does not worsen. [1]

Surgical Preparation

Thorough preparation is critical to reducing the risk of postoperative complications from cataract surgery. [1]

Several products are applied to the eye prior to surgery, including: [1]

  • Medications to dilate the pupil
  • Antibiotics to reduce the risk of infections
  • Anti-inflammatory medications to reduce the risk of uveitis

Some surgical teams also use oral anti-inflammatories to further reduce the risk of uveitis. [1]

Once the horse is ready for surgery, the team induces general anesthesia and places the horse on a surgical table. [1] Most surgeons add strong muscle relaxants to the anesthetic protocol to prevent the eye from moving during surgery. [1]

Some may also apply topical anesthetics to the eye to prevent the eyelids from closing in response to pressure on the eye surface. [1]

Surgical site preparation involves trimming the eyelid hairs and eyelashes and scrubbing the eye area with surgical scrub. [1] The interior surfaces of the eyelids are sanitized using a cotton-tipped applicator. [1]

These procedures further reduce the risk of postoperative infections by reducing the local bacterial load on the skin. [1]

Surgical Techniques

The following contains detailed descriptions of surgical procedures on the eye, which may be sensitive for some horse owners. Reader discretion is advised.

The two main surgical techniques used for cataract removal in horses are aspiration which may be combined with phacofragmentation (breaking the lens). [1]

In both techniques, the surgeon punctures the lens with a large needle and removes the lens contents through a syringe. [1] Phacofragmentation involves using ultrasonic vibrations to break up the lens fibers prior to aspiration, allowing for easier removal. [1] The surgical process completely removes the lens tissue. [1]

In some cases, surgeons may place an intraocular lens implant, which replaces some of the function of the lens. [3] Intraocular implants are relatively new technology in equine ophthalmology, however ongoing studies show promising results for improving visual function after surgery. [9]

Postoperative Care

Cataract surgery requires extensive postoperative care to ensure that the eye heals correctly. [3] Owners should be aware that at least three months of postoperative care is necessary after cataract surgery, which may include administering medications every 4 – 6 hours. [3]

Horses typically require 3 – 4 weeks of stall rest with hand-walking only while their surgical site heals. [10] Veterinarians often recommend using a mask with an eye cup over the affected eye to prevent the horse from injuring or irritating the eye by scratching. [10]

Medications

The goal of postoperative management is to reduce the risk of infections and inflammation, which can cause delayed or incomplete healing of the surgical site. [3]

Medications administered after cataracts surgery may include: [1][3]

  • Topical steroids or anti-inflammatories for around three months
  • Topical antibiotics for around three months
  • Oral anti-inflammatories for 10 – 14 days (i.e. flunixin meglumine)
  • Medications to prevent stomach ulcers due to stress for 28 days (i.e. omeprazole)

Many horses are resistant to having medications applied directly to their eyes. In such cases, the veterinarian will place a subpalpebral lavage system to make it easier to administer medication. [11]

This system involves tubing inserted through the eyelid and then run along the horse’s head and neck. [1] This configuration allows the owner to inject medications into the tubing system and apply them to the eye surface from a distance. [1]

Rechecks

Several rechecks by a veterinarian are also necessary during the healing period after cataract surgery. [1] The general recheck schedule after surgery involves examinations the day after surgery, then at 7 – 10 days, 4 – 6 weeks, 3 months, 6 months, and 12 months after the operation. [1]

Some surgeons also recommend yearly rechecks for the remainder of the horse’s life. [1]

During these examinations, several diagnostic tests are usually performed to ensure that the eye is healing appropriately and that no complications are developing. [3] Diagnostics may include: [3]

  • Ophthalmic examination
  • Tonometry (measuring the pressure of the eye)
  • Electroretinography (measuring the electrical activity of the retina)
  • Ultrasound of the eye

Complications

The most common complication after equine cataract surgery is uveitis (inflammation within the eye). [1] Uveitis can cause scarring within the eye and phthisis bulbi, a condition where there is such significant damage to the eye that it shrinks and becomes entirely non-functional. [1]

Surgeons attempt to reduce the risk of uveitis by ensuring they remove all the lens material, as lens proteins are a major trigger of inflammation. [1] Anti-inflammatory medications applied to the eye and given orally also help reduce the risk of uveitis. [1]

Other complications that can occur include: [1][3][6]

  • Infections of the eye
  • Corneal ulcers: Damage to the eye surface
  • Retinal detachment: Separation of the retina causing complete loss of vision
  • Corneal edema: Swelling of the eye surface
  • Glaucoma: Increased pressure within the eye
  • Globe rupture: Complete rupture of the eye

Prognosis

Most horses have a reasonable prognosis after cataract surgery, with many going on to performance careers. [3] Deciding whether a horse is safe to use as a riding horse depends on several factors: [1]

  • Degree of remaining vision, usually measured by having the horse negotiate obstacles
  • The horse’s demeanor
  • Whether both eyes are affected
  • Rider’s level of experience
  • Complexity of the desired discipline

The reported degree of vision after cataract surgery varies depending on the study and the timeframe examined. [1] A long term study following horses for up to 24 months after cataract surgery showed that 87% of horses retain usable vision for up to one month after the operation. [12]

By 24 months after surgery, only 26% of horses still had measurable vision in the treated eye. [12] Horses with evidence of uveitis prior to surgery are much less likely to retain usable vision after surgery. [12][13]

Although the long-term prognosis for vision appears poor, many horses manage very well with vision in only one eye. [14]

There are one-eyed performance horses in all levels of equestrian sport, including dressage, jumping, racing, and Western events. [14] Horses appear to have reasonable depth perception and can often negotiate obstacles successfully despite their restricted vision. [14]

Intraocular Implants

Currently, the efficacy and success rate of intraocular lens implants in horses are unknown. [3] Some studies suggest that intraocular implants may have a higher rate of postoperative complications using current techniques. [6]

More studies are necessary to identify the most effective type of lens implants, best surgical approaches, and other factors surrounding these medical technologies. [3][11]

Summary

Cataracts are cloudy areas within the horse’s eye lens that can disrupt normal vision.

  • The most common cause of equine cataracts is equine recurrent uveitis
  • Most horses show no symptoms of cataracts unless severely affected
  • The only treatment available is surgical removal of the lens
  • Horses typically have a reasonable prognosis for performance careers after cataract surgery

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References